Severe obesity is associated with a 2 to 10-fold increase in total mortality. Obstructive sleep apnea (OSA) is a condition closely associated with severe obesity. By the application of CT and MRI, several studies have shown that the upper airway of patients with sleep apnea is smaller than that of normal subjects, even during wakefulness. Weight loss can result in clinical improvement in subjects with sleep apnea. Severe obesity is associated with insulin resistance and hyperinsulinemia. Weight loss in severely obese subjects has been shown to be associated with near-normalization of insulin sensitivity. The specific aims of the study are to determine if 3 months of weight loss followed by 3 months reduced-weight maintenance in severely obese subjects will (1) affect overall sleep dynamics; (2) are sufficient to result in morphologic changes in the neck; (3) will show effects on neck morphology and/or sleep dynamics that relate to changes in pulmonary function; (4) will exhibit individual metabolic responses to dietary macronutrient composition (HC vs. HF) which are associated with, or predictive of, changes in sleep dynamics; and (5) will exhibit individual metabolic responses to dietary macronutrient composition (HC vs. HF) which are associated with amount of weight loss and/or predictive of weight regain.